This invention relates to an apparatus for converting rotational motion to linear motion. More particularly, this invention relates to an apparatus for converting rotational motion to linear motion which utilizes a screw shaft and a nut, both having helical threads and between which are interposed threaded planetary rollers.
Prior to the present invention, roller screw drives have been available which comprise a screw shaft axially positioned within a nut and wherein a plurality of planetary rollers are positioned between the screw shaft and the nut. The rollers mesh with a helical thread on the nut and a helical thread on the screw shaft. The helix on the nut and the screw shaft extend in the same helix direction. In addition, the shaft and the nut have the same lead (distance advanced per revolution) and the same number of starts (beginning of a helical thread). When the nut is rotated, the shaft is caused to move axially with respect to the nut and the rollers. The rollers may or may not move axially with respect to the nut. As used herein, the term "start" is defined as the beginning of a helical thread in the right-hand screw thread direction or clockwise moving away from the observer. The term "negative start" is defined herein as the beginning a helical thread in the lefthand screw thread direction or counterclockwise moving away from the observer. Thus, as used herein, a nut and screw will have equal starts when each has the same number of starts and the starts extend in the same direction.
Presently available roller screw drives utilize a nut and a screw having the same number of starts. In order to maintain the planetary rollers having starts within the nut during rotation, the ratio of the nut diameter to the roller diameter is made equal to the number of starts. High gear ratios (small lead) may be obtained only when utilizing a very fine thread design: i.e., a large number of threads per unit length which is impractical to produce due to the very tight tolerances. Furthermore, when utilizing a nut and screw shaft, the threads of which extend in the same helical direction, with planetary rollers having starts, a large helix angle mismatch of the shaft threads with the planetary rollers occurs which results in poor load carrying capability.
When utilizing a nut and screw shaft, the threads of which extend in the same direction with planetary rollers having no starts, it is mandatory that the rollers be recirculated; i.e., that the rollers be removed from engagement with the nut and screw shaft and be moved axially to return the planetary roller to a former position so that the roller will be retained between the nut and screw shaft during extended rotation times. This mode of operation causes radially asymmetrical static forces and radial and axial dynamic imbalances that prevent quick acceleration or deceleration and also prevents high speed rotation.
Accordingly it would be desirable to provide a roller screw drive which permits obtaining a high gear ratio (small lead) without the necessity of utilizing a fine thread design. Furthermore, it would be desirable to utilize a roller screw design which does not require extreme thread helix angle mismatch between the planetary rollers and the nut and screw. Also, it would be desirable to have a roller screw design that utilizes planetary rollers which does not require roller recirculation.